1. Field of the Invention
The present invention relates to an automatic flash control device of a camera, which device divisionally measures flashing light emitted from a flashing means.
2. Background of the Invention
As a conventional device of this type, a device disclosed in Japanese Laid-Open Patent Application No. 3-68928 filed by the present applicant is known.
Before exposure, this device causes a flashing means to perform preliminary flashing, divisionally measures a light beam reflected by an object field and then reflected by a shutter curtain, and determines flashing control conditions for main flashing. Thereafter, during exposure, the device causes the flashing means to perform main flashing, divisionally measures a light beam reflected by an object field and then reflected by a film surface, and performs flash control under the conditions determined in the preliminary flashing mode, thus performing proper exposure.
However, in the conventional device described above, since the photometry operation in the preliminary flashing mode is performed using light reflected by the shutter curtain, and the photometry operation in the main flashing mode is performed using light reflected by the film surface, a photometry area is displaced due to a difference between the positions of the shutter curtain and the film surface, and the identity between the photometry areas in the preliminary and main flashing modes is disturbed. The adverse effect of the displacement on flash control becomes more serious as the photometry area is segmented more. This will be described in detail below.
FIG. 5 is a sectional view of a camera having a conventional automatic flash control device.
In a finder observation mode, a light beam from an object field, which passed through a taking lens 3 and an aperture 4 in a taking lens barrel 2, and was reflected by a main mirror 5 (in a state indicated by a dotted line) in a camera main body 1, passes through a screen 6, a pentagonal prism 7, and an eyepiece lens 8, and reaches the eye of a photographer.
Some light components of the light beam emerging from the pentagonal prism 7 pass through a focusing lens 9 for stationary light photometry, and reach a photometry element 10 for stationary light photometry. The photometry element 10 is a divisional photometry type element for performing a photometry operation while dividing an object field into five areas.
When a photographer depresses a shutter button (not shown), the main mirror 5 escapes from the dotted line position to a solid line position, and the aperture 4 is stopped down in accordance with an exposure value which is calculated based on a photometry value measured by the photometry element 10. Then, a flashing device 11 performs preliminary flashing.
The preliminary-flashing light is reflected by an 10 object field, passes through the taking lens 3 and the aperture 4, is reflected by a curtain of a shutter 12, passes through a focusing lens 13 for flashing photometry, and reaches a divisional photometry type photometry element 14 for flashing photometry. The focusing lens 13 and the photometry element 14 will be described in detail later with reference to FIG. 6.
A photometry area, a correction amount of a photometry level, and the like in the main flashing mode are determined from the photometry result of the preliminary-flashing light.
Then, the shutter 12 is opened, and the flashing device 11 performs main flashing. The main-flashing light beam, which was reflected by an object field and passed through the taking lens 3 and the aperture 4, reaches a film surface 15. Thus, the film is exposed, and some light components reflected by the film surface 15 reach the photometry element 14 via the focusing lens 13.
FIG. 6 is a perspective view showing the details of the focusing lens 13 and the photometry element 14.
The photometry element 14 is divided into five light-receiving portions 14a to 14e, and performs a photometry operation while dividing the film surface 15 into five areas 15a to 15e, as shown in FIG. 6.
The focusing lens 13 is constituted by three lenses 13a to 13c. The lens 13a is arranged to allow the light-receiving portion 14a to measure the area 15a, the lens 13b is arranged to allow the light-receiving portions 14b and 14d to measure the areas 15b and 15d, and the lens 13c is arranged to allow the light-receiving portions 14c and 14e to measure the regions 15c and 15e.
FIG. 7 is a view showing a flashing photometry optical system. Assume that the angle between an optical axis 16 of the focusing lens 13 and an optical axis 17 of the taking lens 3 is represented by .theta., and a crossing point A between the two optical axes is defined as the central position on the film surface. If the shutter curtain 12 is located in front of the film surface 15 to be separated by a distance d1, a crossing point B between the optical axis 16 and the shutter curtain 12 is displaced toward the bottom side by a distance d2 from the optical axis 17. The distance d2 is given by: EQU d2=d1.times.tan .theta.
Since, in general, d1=3 mm and .theta.=45.degree., d2 is about 3 mm.
Therefore, the photometry area in a shutter curtain photometry mode indicated by a dotted circle and the photometry area in a film surface photometry mode indicated by a solid circle are displaced from each other by about d2=3 mm.
This displacement poses a very serious problem when the photometry operation is performed while dividing the central position finely, as shown in FIG. 9. Central small area portions are used in photometry operations of different areas in the shutter curtain photometry mode and in the film surface photometry mode.